One approach to the problem has been to form the pistons of such actuators of brittle material which is strong enough for normal operation but having weak sections which break away if forced against an internally projecting deformation of the cylinder. This permits the rod and piston to be moved past the damaged part of the cylinder. Another design uses frangible layers of plastic material on the interior surface of the cylinder and/or the exterior surface of the rod such that inward deformation or "petalling" of the metal cylinder will, in most cases, be less than the thickness of the plastic layer. Hence the piston, which is sized to the interior of the plastic layer, will pass even though the actuator is inoperative. Should a projectile penetrate to the point of impacting on the rod, deforming the rod, it will also break away the plastic layer on the outside of the rod. Since the port in the rod end gland is sized to the plastic layer on the rod, the rod will pass through the end gland even if deformed substantially.
The above described designs are useful and effective if the actuator is sufficiently large to provide clearance for the broken away or deformed parts. Many small actuators are used, however, in which the clearance between the cylinder bore and the piston rod diameters is quite small, leaving insufficient space to accommodate broken away or deformed metal parts. The presence of such deformed or broken parts may result in requiring excessively high loads to unjam the actuator. There is, therefor, a need for a design for small actuators which will enable them to avoid jamming if hit by ballistic fire.